1. Field of the Invention
The present invention relates in general to the sequestration of ammonia, and more particularly to the process for removing ammonia from a liquid or gaseous medium by contact with a crystalline molecular sieve having certain physical and chemical properties hereinafter elucidated. The invention also relates to molecular sieve compositions suitable for use in the process.
2. Description of the Prior Art
Ammonia is a colorless gas with a pungent odor which in small concentrations in the atmosphere is merely objectionable for its assault on the olfactory sense organs, but in larger concentrations is toxic and harmful to animal tissue. Ammonia in concentrations as low as 20 ppm (volume) is detectable by the human sense of smell, and at concentrations of as low as 100 ppm becomes irritating as an odor and can bring tears to the eyes. Ammonia gas is extremely soluble in water particularly at ambient room temperature and below. At 0.degree. C for instance, one volume of water dissolves 1298 volumes of gaseous ammonia. At 20.degree. C a lesser but still substantial 710 volumes of the gas are dissolve. Whereas hazardous levels of ammonia presence are ordinarily encountered in industrial environments, the merely objectionable, i.e. malodorous, concentrations of ammonia are usually encountered as the product of bacterial action on nitrogeneous organic matter, particularly animal excretia.
Being a highly polar compound, the adsorption of ammonia, either from gas streams or aqueous media, has long been carried out using a wide variety of adsorbent media. Commonly employed solid adsorbents include activated charcoal or active carbon, silica gel, activated alumina, kieselguhr, fullers earth and other clay minerals, and zeolites of both the crystalline and the amorphous types. A mixture of alfalfa, bentonite clay and a binder is proposed in U.S. Pat. No. 3,789,799 for adsorbing and neutralizing odors of animal waste matter. In U.S. Pat. No. 4,437,429, the use of a hydrated zeolite in admixture with clay is proposed as being particularly useful for the control of odors from pet litter, it being observed that the use of zeolites by themselves as litter material has generally been unsuccessful due to their poor water adsorption properties as compared with clays. For the adsorption of certain odors from animal litter using a mixture of clays and zeolites, it is proposed in U.S. Pat. No. 4.437.429 that the zeolite constituent not only be employed in its hydrated state, but also that the water of hydration be the original water of hydration. It is said to be not sufficient that water is added to a previously heat-treated zeolite from which the original water of hydration was driven off.
In general, when zeolites have been utilized for the suppression of ammonia or other odors, the preferred species have been those with a low framework Si/Al ratio and high degree of adsorption capacity for water or other highly polar molecules such as ammonia or hydrogen sulfide. The disclosure of the aforesaid U.S. Pat. No. 4,437,429 is somewhat remarkable for its specific disclosure of may of the class of so-called high silica zeolites which are synthesized using organic templating agents such as the tetraalkylammonium ions. These include ZSM-5. ZSM-11, ZSM- 12 and ZSM-23. The preferred zeolite species is the natural mineral clinoptilolite, however, which has a nominal framework SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of about ten. It has also been proposed, U.S. Pat. No. 4,648,977, to use high-silica molecular sieves, including the silica polymorph silicalite, to adsorb toxic organic materials, from aqueous media for water purification purposes, but for the removal of dissolved ammonia it has been the common practice to utilize high-alumina zeolites such as clinoptilolite, eronite, mordenite, ferrierite, zeolite L, zeolite T, zeolite W, zeolite F and synthetic mordenite, for two reasons. Firstly, the large number of cationic sites in the high alumina zeolites increased the number of cations which could be ion exchanged with ammonium ions, ion exchange being considered the principal mechanism involved. Secondly, the adsorptive capacity of zeolites for molecules having a high permanent dipole moment was believed to be enhanced by the electromagnetic environment within the zeolite void space produced by the arrangement of the cation sites of the zeolite structure.